The circle of Willis is situated at the base of the brain and gives off to the front the two anterior cerebral arteries, to the sides the two middle cerebral arteries, and to the back the two posterior cerebral arteries. The two anterior cerebral arteries are connected by the anterior communicating branch, and the middle cerebral artery and the posterior cerebral arteries on each side are connected by the posterior communicating branches. The cerebral arteries terminate in the piamater as a dense capillary network, and from there supply the substance of the brain with nutrition.
Fig. 30—The circle of Willis. (Spalteholz)
The external carotid artery supplies the muscular tissues of the face. The external carotid artery arises from the common carotid artery at about the level of the upper border of the thyroid cartilage—a level which corresponds with the body of the fourth cervicle vertebra. Thence it is directed upward and slightly backward towards the angle of the jaw, where it enters the substance of the parotid gland and continues upward in that structure to just below the root of the zygoma. Here it gives rise to a large branch, the internal maxillary, and is then continued upward over the root of the zygoma upon the side of the skull, this terminal portion of it being termed the superficial temporal artery. The branches of the external carotid artery from below upward are (1) the ascending pharyngeal, (2) the superior thyroid, (3) the lingual, (4) the occipital, (5) the facial or external maxillary, (6) the posterior auricular, (7) the internal maxillary, (8) the superficial temporal.
Fig. 31—The arteries of the face and scalp. (Gray)Fig. 32—The external carotid and its branches. (Gray)
The arch of the aorta now continues into the thoracic aorta, so called while it is in the thoracic cavity, and after it has passed through the diaphragm becomes the abdominal aorta. At a point opposite the umbilicus or navel the abdominal aorta divides into the two common iliac arteries. Each common iliac artery divides into an internal iliac artery, which supplies the organs of the pelvic cavity, and an external iliac artery, which passes beneath Poupart's ligament. As the artery passes down the leg it is known as the femoral artery, until it passes into the popliteal space, where it is called the popliteal artery. About one inch below the popliteal space the artery divides into the anterior tibial artery, which runs on a straight line down the front and outside of the leg to a point between the big toe and the one next to it, and the posterior tibial artery which passes down the back part of the foreleg between the inside ankle and the heel. The peroneal, a branch of the posterior tibial, passes down the foreleg between the outside ankle and the heel. The anterior tibial artery, as it passes through the instep is known as the large dorsal artery and further on is known as the small dorsal artery. In the foot is the plantar arch, formed by branches of the posterior and anterior tibial arteries, which send out branches to each toe.